Percussion tool bits are employed for drilling rock, for instance as in drilling wells, or more frequently for drilling blastholes for blasting in mines and construction projects. The bits are connected to a drill string at one end and typically have a plurality of cemented tungsten carbide inserts embedded in the other end for drilling rock formations. In use, the drill string and bit body are moved up and down rapidly, striking the rock being drilled in a percussive motion. A typical air hammer for percussion bits operates at about 2,000 blows per minute and rotates at about 60 r.p.m. Some percussion bits are driven through hydraulic action. The percussion bit hammers the inserts against the rock being drilled, shattering it by repeated blows. Compressed air pumped through the bit removes chips of fractured rock from the bore hole being drilled. Such bits range in size from 75 millimeters to more than 25 centimeters in diameter.
Rock bits wear out or fail in such service after drilling many meters of bore hole. The cost of the bits is not considered so much in the cost of the bit, per se, as much as it is considered as the cost of drilling per length of hole drilled. It is considered desirable to drill as much length of bore hole as possible with a given bit before it is used to destruction. It is also important that the gage diameter of the holes being drilled remain reasonably near the desired gage. Thus, wear of the bit that would reduce the hole diameter is undesirable. Further, wear of the inserts in the bit during drilling reduces their protrusion from the surface of the steel bit body. The protrusion, has a strong influence on the drilling rate. Thus, as the inserts wear out, the rate of penetration may decrease to the extent that it becomes uneconomical to continue drilling. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation, both for reducing bit costs and for maintaining a reasonable rate of penetration of the bit into the rock.
Heavy duty rock bits are employed for drilling wells in subterranean formation for oil, gas, geothermal steam and the like. Such bits have a body connected to a drill string and a plurality typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the body at its lower end. In use the drill string and bit body are rotated in the bore hole and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. As such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 61/2 to 121/4 inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. The other principal reason for replacing a rock bit on a drill string is that the bearings supporting one or more of the roller cones fail due to excessive wear, loss of lubricant or the like. There are a variety of other minor causes of failure in rock bits in some rock formations. There is a continual effort to upgrade the performance and lengthen the lifetime of those components of a rock bit that are likely to cause a need for replacement. There have been continual improvements in bearings, but few major improvements in the cutting structure of rock bits.
When a rock bit is drilling a bore hole, it is important that the diameter or gage of the bore hole be maintained at the desired value. The outermost row of inserts on each cone of a rock bit is known as the gage row. This row of inserts is subjected to the greatest wear since it travels furthest on the bottom of the hole, and the gage row inserts also tend to rub on the sidewall of the hole as the cones rotate on the drill bit body. As the gage row inserts wear, the diameter of the bore hole being drilled may decrease below the original gage of the rock bit. When the bit is worn out and removed, a bottom portion of the hole is usually under gage. When the next bit is run in the hole, it is therefore necessary to ream that bottom portion of the hole to bring it to the full desired gage. This not only takes substantial time but commences wear on the gage row inserts, which again results in an under gage hole as the second bit wears out. Further, as the bit reams, a side load is applied to the cones, "pinching" the bit and applying a high side load on the bearings, which can cause premature failure of the bearings.
The rate of penetration of a rock bit into the rock formation being drilled is an important parameter for drilling. Clearly, it is desirable to maintain a high rate of drilling since this reduces the time required to drill the bore hole, and such time can be costly because of the fixed costs involved in drilling. The rate of penetration decreases when the inserts in the bit become worn and do not protrude from the surface to the same extent they did when drilling commences. The worn inserts have an increased radius of curvature and increased contact area on the rock. This reduces the rate of penetration.
Thus, it is important to maximize the wear resistance of the inserts in a rock bit to maintain a high rate of penetration as long as feasible. It is particularly important to minimize wear of the gage row inserts to maximize the length of hole drilled to full gage.
Wear resistance of conventional inserts of cemented tungsten carbide may be enhanced by increasing the proportion of tungsten carbide and decreasing the proportion of cobalt in the composite material. This increases the hardness and wear resistance of the cemented tungsten carbide but reduces its toughness so that the inserts are more susceptible to breakage than inserts with higher cobalt content. In exemplary embodiments, the cobalt content of inserts for use in rock bits ranges from about six percent to sixteen percent by weight cobalt.
Another factor that influences wear resistance and toughness is particle size of the tungsten carbide phase. Exemplary particle size in an insert is in the range of from three to seven microns. This particle size is an average particle size of a powder mixture that includes larger and smaller particles. For example, when the average particle size is five microns, there are submicron size particles present as well as particles as large as seven or eight microns. Generally speaking, toughness increases with larger particle size and so does wear resistance. A common grade of cemented tungsten carbide for rock bit inserts has an average tungsten carbide particle size of about six microns and contains about ten to fourteen percent by weight cobalt.
Toughness of the inserts is important in a rock bit since the inserts are subjected to impact loads, as well as wear by rubbing against the rock formation. Breakage of inserts can be a substantial problem since it not only results in reduced drilling activity, but the fragments of a broken insert may damage other inserts. It is therefore desirable to provide inserts that are hard, to resist wear, and tough, to resist breakage.